The ultrastructure of the extraocular motor units will be studied in the adult rat trochlear nucleus, nerve, and superior oblique muscle. Measurements and the pattern of various components of the motor unit will be examined by light and electron microscopy to test the neuron size-principle for slow and fast units. This will be directly compared to the classification and histochemical profile of the muscle fibers using a variety of staining methods. The muscle will also be examined for sensory nerves and multiple motor endplates. The role of polyneuronal innervation and the developing synaptic pattern of the trochlear neurons during postnatal differentiation will be studied by light and electron microscopic methods. The effects of monocular sensory deprivatio on the trochlear motor nerve will also be studied by morphometric analysis. The experimental methods of intramuscular and midbrain injections of the protein tracer horseradish peroxidase will be used to evaluate the sensory innervation, size principle and interneurons with respect to retrograde axoplasmic flow and the trophic interactions involved. The morphological specificity of the neuromuscular components in retrograde transport of tracer molecules will be related to possible control mechanisms of eye movements.